1. Field of the Invention
The present invention relates to devices for detecting unit failures in a polyphase system, and more particularly, to a device for detecting capacitor unit failures in a multi-phase grounded wye-connected capacitor bank in a high voltage transmission system.
2. Description of the Prior Art
Various polyphase systems are used by electrical utilities in high voltage transmission systems. For example, three-phase multi-capacitor wye-connected capacitor banks are used by electrical utilities for power factor correction and voltage regulation in three-phase high voltage transmission systems. These capacitor banks are typically made up of three single-phase legs, each leg made up of groups of capacitor units connected in series, where each group consists of one or more individual capacitor units connected in parallel. One side of each leg is connected to one of three transmission lines of the three-phase high voltage transmission system. The opposite side of each leg of the capacitor bank is connected to a ground point. This arrangement is commonly known as a "grounded-wye" capacitor bank. Such capacitor banks are extremely beneficial in maintaining voltage, reducing losses, reducing operating costs, and delaying the need for building additional transmission lines.
However, several problems have been experienced by such capacitor banks, one of which is commonly known as a "cascading" failure. Typically, each capacitor in the bank is individually fused, and the loss of a single capacitor within a series group with the attendant fuse opening increases the impedance of that capacitor group thereby increasing the voltage across the group and increasing the possibility of failure of the remaining capacitors in its particular group. This increased voltage has, in some cases, caused sufficient over-voltage to result in complete failure of the remaining capacitors in the entire phase leg of a three phase bank.
The sensitivity of capacitor units to over-voltage is well known. The operating life of a capacitor is normally very long provided there is no application of voltage over the rated voltage of the capacitor. Typically, over-voltage of more than 110% of the rated voltage of the capacitor unit may cause failure or drastically reduce its life expectancy. Thus, loss of as few as one capacitor in a group of capacitors in a capacitor bank may be sufficient to increase the voltage on the remaining capacitors to a level sufficient to cause successive failures of the other capacitors. It is this "cascading" effect which makes the problem of detecting the loss of an individual capacitor unit important to the overall protection of the capacitor bank.
Many sensing systems and capacitor bank configuration designs have been utilized in an attempt to avoid the cascading effect by sensing the loss of individual capacitor units. One means of protecting grounded wye capacitor banks has been by use of a current transformer to sense the current from the neutral point to ground. However, energization of a grounded wye capacitor bank is unavoidably accompanied by extremely high in-rush currents between the bank neutral and ground, particularly when parallel banks are already energized. Such in-rush currents can be of the magnitude of thousands of amperes thus requiring adequate surge protection of the current transformer itself and the sensing equipment connected to the secondary of the sensing transformer. Adequate surge protection is expensive and difficult to achieve. Further, this scheme usually lacks sufficient sensitivity to provide proper protection since the selection of a suitable current transformer to provide proper sensitivity usually leads to current transformer and/or control equipment damage or nuisance tripping of the capacitor bank during high in-rush currents. To avoid this, a current transformer of a higher ratio is selected which decreases the sensitivity of the protection. Further, this arrangement does not provide for compensation for unbalance due to capacitor unit manufacturing tolerance variations or fixed system voltage unbalance.
Another prior art scheme of the type disclosed in U.S. Pat. No. 3,181,031--Yee uses six potential transformers (two per phase) and an over-voltage relay. For each phase, one potential transformer senses line voltage, the other senses the voltage at an intermediate tap point within each phase leg. However, the tap point normally is selected just ahead of the series group that is nearest to ground potential. With this sensing arrangement, the increased impedance of any series group above the tap point due to loss of capacitor units will not effect the tap point voltage by as large a percentage as the loss of capacitor units below the tap point. Hence, the loss of capacitor units below the tap point may cause nuisance tripping, or alternatively, an unnecessarily large number of capacitor units above the tap point may fail before lockout level is reached.
An additional problem incident to prior art devices utilized to detect failure of one or more capacitors in a capacitor bank has been the inability of the prior art devices to distinguish between the voltage unbalance caused by power system voltage unbalance or by manufacturing tolerance errors in individual capacitors in the capacitor bank. In larger capacitor banks, the unbalance voltages can produce significant errors or even obscure the signal created by the loss of an individual capacitor unit.
Other polyphase equipment banks have component failure detection problems substantially similar to that of high voltage wye capacitor banks. For example, in many high voltage transmission installations, grounded wye reactor banks comprising air core inductor coils are often connected to the transmission lines and interconnected at a common neutral point. Failure of one of the coils of a reactor bank typically results from the shorting across one or more turns of the layers of the coil. Such failure produces voltage unbalance because of the change of the reactance of the coil. Further, failure to detect the shorting of a winding will ultimately result in catastrophic failure of the inductor.
Thus, the reactor bank has the same detection problems present in a capacitor bank. In particular, manufacturing tolerance error and system unbalance can cause error signals to be introduced which will obscure the detection of component failure.
Accordingly, it would be a highly desirable advance in the art to provide an apparatus for detecting failure of one or more components of a polyphase equipment bank.